Lasers Measure Gas Emission Levels

Star Wars-type technology is being used to more accurately determine the origin and concentration of specific manure gases.

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Star Wars-type technology is being used to more accurately determine the origin and concentration of specific manure gases.

Manure lagoons give off ammonia. Everyone who has a sense of smell knows it. But measuring exactly how much ammonia a lagoon emits is a challenge.

The human nose is able to detect ammonia levels at around 20 parts per million (ppm), but will lose sensitivity with long exposure.

In conjunction with Alberta Agriculture, Sean McGinn, a researcher at Agriculture and Agrifood Canada's Lethbridge Research farm, has taken on the challenge of accurately measuring gas emissions from livestock facilities — and he's using laser technology to do it.

“The lasers can be tuned to measure concentrations of specific gases,” McGinn explains. “If you want to measure an invisible, odorous gas like ammonia, you start by tuning the laser to a frequency that will react with it. When it's tuned to this specific wavelength, it will agitate any target compound molecule it encounters and use up some of its energy.

“The process is similar to how microwaves selectively agitate water molecules to warm food,” he continues. “A detector then measures this decrease in energy, letting you determine the number of molecules of the specific gas the laser beam encountered along the line of concentration.”

Knowing the concentration of a specific gas in a particular area is only part of the equation, however. The next step is determining how much of the gas is coming from a particular source, such as your or your neighbor's pit or lagoon, or a nearby municipal sewage lagoon.

There are two ways to determine a gas's point of origin. “One is called the mass balance approach. We set up a laser at the upwind edge of the source to determine the amount of incoming gas, and then measure the amount of concentration leaving the lagoon (on the downwind side). The difference between the incoming and outgoing ammonia would be the amount that was generated by the lagoon itself,” McGinn explains.

“Another more novel technique is known as the dispersion model. This actually simulates what the plume of gas coming off the source looks like. By monitoring the concentration within that plume, and understanding what the characteristic of the plume is, we calculate backwards to figure out the emission rate of that source. This model is quite nice. It can be operated remotely from the source,” he explains. “For example, we plan to use open-path lasers downwind of a feedlot so we can take measurements without disrupting the operation of the feedlot.”

Plumes of invisible gases, like ammonia, act very similar to visible plumes, such as smoke. The plume gets bigger and more diluted the farther it drifts. The more it is diluted, the less pronounced the odor.

“The plume's shape is a factor of the stability of the air,” McGinn says. “For example, during the daytime, air near the soil surface is considered unstable due to daytime heating. At night, we quickly move into more stable air as the soil surface cools. Variables like stability have to be characterized when we do dispersion modeling.”

Accurate Measurements Are Important

“Once you know what the actual emission rate from a source is, you can look at what factors might mitigate it,” McGinn says. “For example, some work out of Denmark has shown a ten-fold variation in the emission rates from slurry tanks, but it doesn't give us any indication of what is causing it. Our continuous measurement technique using lasers allows us to look at the effects of variables such as wind speed, trash on the lagoon surface or the effect surface temperature has on emissions.”

McGinn's work shows there is a strong correlation between the concentration of ammonia and wind speed downwind of a lagoon. “The windier the neighborhood you live in, the less problems you are going to have with odor,” he says. “This has implications when applying manure to fields. If you apply manure to fields under windy conditions, you can disperse odors more readily. But, at the same time, you increase the loss of nitrogen overall, because more ammonia will be lost and dispersed.”

In another type of experiment, McGinn uses lasers to look at emissions from animals in chambers. “These measurement techniques let us look at the effect that different diets have on methane emissions,” he says. “They also give us an accurate way to quantify emission rates and to put them into perspective with other sources. For example, some of the emission inventory with methane has shown that swine produce very little greenhouse gas relative to the beef industry. It allows us to compare different sectors and shows where the emphasis should be in terms of mitigating greenhouse gases.”